PARIS—French President François Hollande paid tribute Monday to the four men who foiled an apparent terrorist attack on a high-speed train, saying their actions prevented a “massacre.”

During a ceremony at the Élysée Palace, the French leader awarded the Legion of Honor—France’s highest civilian decoration—to Airman First Class Spencer Stone, Oregon National Guard member Alek Skarlatos, college student Anthony Sadler and Christopher Norman, a U.K. national.